This invention relates to improvements in a production method of a cam lobe piece of an assembled camshaft which functions as a main element in a valve operating system for an internal combustion engine, and more particularly to the production method of the cam lobe piece of the assembled camshaft arranged such that the cam lobe piece as a forging is fixedly mounted on a hollow shaft upon diametrical expansion treatment of the hollow shaft.
The cam lobe piece of the assembled camshaft is conventionally formed of a sintered material or a forging. In case of the cam lobe piece formed of the forging, a high carbon steel (for example, S70C or S55C according to Japanese Industrial Standard) has been used as the material for the cam lobe piece in order to particularly obtain a high surface hardness. The forging upon being forged is subjected to hardening so as to be used as the final product of the cam lobe piece. In general, the cam lobe piece of the forging is formed under hot forging excellent for forming the cam lobe piece as disclosed in Japanese Patent Provisional Publication Nos. 9-276976 and 9-280013.
Now, the built-up camshaft is assembled by press-fitting a pipe-shaped shaft into the shaft bore of the cam lobe piece. At this time, a press-fit pressure and a assembly precision between the shaft and the cam lobe piece are ensured by a press-fit amount. Consequently, a high precision is required for the outer peripheral dimension of the shaft and the inner peripheral dimension of the cam lobe piece. However, in case of the forged cam lobe piece formed by the hot forging using the high carbon steel as the material, production of oxide scale and thermal shrinkage occur in the forging during the hot forging, thereby inviting dimensional change of the forging. Thus, the forged cam lobe piece cannot obtain a sufficient dimensional precision required for a part of the assembled camshaft. In view of this, in order to obtain a required inner peripheral dimension of the cam lobe piece, it is required to apply finishing such as cutting (for example, broaching) or cold plastic working onto the formed cam lobe piece at a separate step. This increases the number of steps and man-hour for managing intermediate stocks, thus inviting cost-up in production.
Additionally, in case of the forged cam lobe piece formed of the high carbon steel, the formed cam lobe piece is required to be subjected to hardening in order to secure its surface hardness, in which quenching crack may occur. For the particularity of the material itself, it is impossible to completely get rid of the quenching crack during the hardening. As a result, inspection for judgment as to whether the quenching crack has occurred or not and selection for the hardened products having the quenching crack are required in order to previously prevent occurrence of damage during a press-fitting assembly and insufficient press-fitting pressure due to the quenching crack. This lowers yield of the product and increases the number of steps in production, thereby further contributing to the cost-up in production.
In view of the above, a production method of the cam lobe piece employing cold forging as a basic working has been proposed in place of that employing the hot forging, as disclosed in Japanese Patent No. 2767323.
However, the cold forging is low in forgeability of the material (flowability of the fillet of the material) as compared with the hot forging, and therefore not only defects such as underfill tend to occur but also a forming load applied to a die unavoidably increases if a deformed amount of the material is sufficiently decreased during plastic deformation made from the material to the required product, thereby making wear of the die severe thus contributing to shortening the life of the die.
Particularly in case that a solid cylindrical material is axially upset and compressed, the material is bulged radially outwardly in equal amounts throughout its outer periphery, and therefore it is relatively easy to form the material into a simple circular shape or the like. However, it is difficult to directly form the material into a particular shape which is obtained by synthesizing a base circle section and a rounded projected section (having a notably small radius of curvature as compared with the base circle section) serving as a cam nose in the product, without occurrence of underfill. As a result, it is required to increase the number of steps for production so as to make plastic deformation from the material to the product little by little throughout the increased number of steps. This not only requires the forging facility of the large-size and the high cost but also prolongs time required for working thereby contributing to lowering in productivity.
It is, therefore, an object of the present invention to provide an improved production method of a cam lobe piece of an assembled camshaft, which can effectively overcome drawbacks encountered in conventional production methods of the cam lobe piece.
Another object of the present invention is to provide an improved production method of a cam lobe piece of an assembled camshaft, by which the cam lobe piece of a high precision can be produced without occurrence of its underfill and by a small number of production steps though employing a cold forging as a premise.
An aspect of the present invention resides in a method of producing a cam lobe piece of an assembled camshaft. The method comprises (a) forming a profile of the cam lobe piece by upsetting a material in a direction of thickness of the cam lobe piece under forging to obtain an intermediately formed body; (b) piercing a central portion of the intermediately formed body to form a shaft bore in the intermediately formed body; and (c) ironing an inner peripheral surface of the pierced intermediately formed body to form unevenness at the inner peripheral surface. In the method, the forming the profile of the cam lobe piece, the piercing the central portion of the intermediately formed body and the ironing the inner peripheral surface of the pierced intermediately formed body are accomplished by cold working. Additionally, the material at the forming the profile of the cam lobe piece has a shape including first and second side surfaces which are opposite to each other in the direction of thickness of the cam lobe piece. The first side surface includes first and second surface portions which are substantially parallel with the second side surface. The first surface portion forms part of a first section located on a side of a cam nose of the cam lobe piece. The second surface portion forms part of a second section which is located longitudinally opposite to the first section. The first surface portion is farther from the second side surface than the second surface portion so that a thickness of the material gradually increases in a direction from the second section to the first section.
Another aspect of the present invention resides in a method of producing a cam lobe piece of an assembled camshaft. The method comprises (a) forming a profile of the cam lobe piece by upsetting a material in a direction of thickness of the cam lobe piece under forging to obtain an intermediately formed body; (b) piercing a central portion of the intermediately formed body to form a shaft bore in the intermediately formed body; and (c) ironing an inner peripheral surface of the pierced intermediately formed body to form unevenness at the inner peripheral surface. In the method, the forming the profile of the cam lobe piece, the piercing the central portion of the intermediately formed body and the ironing the inner peripheral surface of the pierced intermediately formed body are accomplished by cold working. Additionally, the material to be supplied for the forming the profile of the cam lobe piece has a section corresponding a cam nose of the cam lobe piece. The section having a rounded end portion has a radius of curvature substantially equal to that of a rounded end portion of the cam nose of the cam lobe piece. The radius of curvature of the material is formed prior to the forming the profile of the cam lobe piece.
A further aspect of the present invention resides in a method of producing a cam lobe piece of an assembled camshaft. The method comprises (a) forming a profile of the cam lobe piece by upsetting a material in a direction of thickness of the cam lobe piece under forging to obtain an intermediately formed body; (b) piercing a central portion of the intermediately formed body to form a shaft bore in the intermediately formed body; and (c) ironing an inner peripheral surface of the pierced intermediately formed body to form unevenness at the inner peripheral surface. In the method, the material has a first section located on a side of a cam nose of the cam lobe piece, and a second section longitudinally opposite to the first section. Additionally, each of the forming the profile of the cam lobe piece, the piercing the central portion of the intermediately formed body and the ironing the inner peripheral surface of the pierced intermediately formed body is carried out in a condition where the first section of the material is located below relative to the second section of the material under a cold working and by using a multi-stage former in which compressive forces are applied laterally to the material.